The use of ion implantation of various elements is well known for improving the wear, friction, and other surface properties of many metal alloys. See "Surface Modification of Metals by Ion Beams", Elsevir Sequoia (1984). For alloys containing primarily the elements cobalt and chromium, both ion implanted nitrogen and titanium have been shown to improve friction and wear properties. See "Friction and Wear Behavior of Cobalt-Based Alloy Implanted with Ti or N", Mat. Res. Soc. Syrup. Proc. 27, p. 637 (1984). For orthopaedic surgical implants, cobalt-chrome alloy implanted with nitrogen has been found to improve the corrosion and subsequent wear/friction properties of the prosthetic joint. See "Medical Applications of Ion Beam Processes", Nuc. Inst. and Meth. in Physics Res. B19/20, pg. 204-208 (1987). Further, the idea of using an ion beam and physical vapor deposition simultaneously bombarding a metal surface has been in use for flat substrates. See "Properties of Aluminum Nitride Films by an Ion Beam and Vapor Deposition Method", Nucl. Inst. and Meth. in Phys. Res. B39, p. 178 (1989).
Ion implantation of nitrogen does produce some improvement in the wear and friction properties of those alloys containing predominantly cobalt and chromium when rubbing against ultra-high molecular weight polyethylene (UHMWPE) in a laboratory test, such as a pin-on-disk wear test. See. U.S. Pat. No. 5,123,924, Sioshansi et al. However, it is possible to further benefit these properties by replacing the metal surface with a continuous surface layer consisting of a ceramic material. Zirconium oxide (ZrO.sub.2 or zirconia) has been found to be beneficial. See U.S. Pat. No. 5,037,438, J. Davidson et al. See also "Low Wear Rate of UHMWPE Against Zirconia Ceramic (Y-PSZ) in Comparison to Alumina Ceramic and SS316L Alloy", J. of Biomed. Mat. Res. 25, p. 813 (1991). Zirconium ions have been ion implanted into iron and steel to improve the corrosion properties. See "Surface Modification of Iron and Steel by Zirconlure or Yttrium Ion Implantation and Their Electrochemical Properties", from "Surface Modification of Metals by Ion Beams 7", Eds. F. A. Smidt, G. K. Hubler, and B. D. Sartwell, Elsevier Sequoia S. A., p. 1 (1992).
The method of growing a zirconium oxide coating on orthopaedic devices described in U.S. Pat. No. 5,037,438 requires that the prothesis be fabricated from pure zirconium metal, which is a costly process. The zirconium oxide is formed by diffusing oxygen gas into the metal at a high temperature in a furnace for a suitable period of time. The process occurs at atmospheric pressure and results in the chemical reaction of the zirconium metal workpiece and oxygen to form a surface layer of zirconium oxide.